That Mesaana will be captured or killed by the Seanchan during their attack under normal circumstances. Of course it isn't completely impossible, especailly knowing the Forsaken's usual "competence", but she would be extraordinary stupid if she stays in the WT till the announced Seanchan attack (it's even mentioned in the Dark Prophecies). In my view it makes anyway the most sense that Mesaana isn't living in the WT anymore since shortly after the episode with SH in CoT, because she feared that Alviarin might recognize her.

Otherwise, if Mesaana has nevertheless still an identity in the WT for some reason, she might be captured by the BA Hunters and Egwene, if Alviarin finally recalls who Mesaana reminded her of and if Alviarin is captured by the Hunters before (which seems quite possible to me). In this case, Messana might of course fall in the hands of the Seancjan, though I think the Forsaken who will already be punished by the Seanchan is obviously Semirhage. Two Forsaken held by the Seanchan seems a bit too much.

Therefore I think most likely Mesaana will simply be killed during TG by someone on the Lightside.